There have been many cases at a construction or building site where the earth must be deeply excavated to form a hole having a depth which is too long relative to its diameter. For example, there have been cases for excavating the earth to form a hole in which an anchor supporting a steel tower is embedded, a hole in which a water purifier tank is embedded, a hole for ground making and a hole for well sinking. In such cases, the hole should generally have a depth which is too long, e.g. ranging from 15 m to 20 m, relative to its diameter, e.g. about 5 m.
In deep excavating work, there is conventionally employed a deep excavator having a telescopic mechanism comprising a stretchable arm arrangement fixed to a boom and wherein a clamshell bucket (hereinafter referred to as a bucket) is coupled to the tip end of a top arm of the stretchable arm arrangement. In the conventional deep excavator, the stretchable arm arrangement is typically fixed to the tip end of the boom and has at least two stages of arms in which the bucket suspended from the top arm is hung to reach the bottom of the hole.
In the conventional mechanism for extending and contracting each arm, a wire or chain is entrained around or extended between each arm whereby each arm is extended and contracted synchronously with one another by such wire or chain. In such a mechanism, it is possible to smoothly extend or contract each arm of the stretchable arm arrangement but the wire or chain must be entrained around or extended to each arm, which makes the arrangement of the wire or chain complex. Furthermore, since the wire or chain for contraction of each arm as well as extension of each arm must be entrained around or extend to each arm, at least two wires or chains are required for one arm, which leads to a complex arrangement of wires or chains. In such an arrangement, the wires or chains are liable to be exposed outside the stretchable arm which is not preferable in view of external appearance. There is also a likelihood that earth or sand becoming stuck to the wires or the chains, which causes abrasion or is troublesome to the mechanism.
Accordingly, there is proposed a mechanism for extending or contracting a stretchable boom using hydraulic power generated by a single hydraulic cylinder which is incorporated into the stretchable arm comprising a plurality of arms. However, in this mechanism the amount of extension of the stretchable boom is limited and the speed of extension is not increased. To solve these problems, there is further proposed a mechanism having two stretchable arms each having a hydraulic cylinder wherein the hydraulic cylinders are simultaneously operated to thereby extend and contract the entire stretchable arm assembly. However, if a plurality of hydraulic cylinders are accommodated in the stretchable arm, it is necessary to provide high pressure application hoses on each hydraulic cylinder coupled to each arm, which makes the mechanism complex.
To solve the problem, there has been proposed a mechanism, for example, as disclosed in my copending U.S. Ser. No. 08/030,844 for simultaneously operating three arms of a stretchable arm using a working unit comprising two hydraulic cylinder units which are assembled and arranged in parallel with each other and structured so that cylinder rods thereof are disposed to operate in opposite directions, wherein the hydraulic cylinder housings are fixed to a middle arm and each of the cylinder rods are coupled to one of the upper and lower arms. In this mechanism, since two hydraulic cylinder units can be operated, there are such advantages that an oil pressure hose is not necessary to be disposed complexly, and the extending and contracting speeds are increased. However, according to this mechanism, the telescopically assembled aim is limited to three arms which are operable simultaneously and cannot cope with more than three arms.
As mentioned above, according to the prior art deep excavator using the wire or chain, it takes substantial time for the bucket to reach the bottom of the hole, which obstructs the effective operation. In the new proposed arrangement using the working unit having two hydraulic cylinder units, since the two hydraulic cylinder units are fixed, there is a disadvantage in that only three arms can operate simultaneously. In this latter arrangement, there are advantages that the operating speed is increased and the trouble is decreased since the wire or chain is not used but there is a disadvantage that the depth of earth to be excavated from the ground is limited since the number of arms is limited to three. Under the circumstances, it is desired to realize an arrangement capable of operating fast, and capable of extending and contracting a stretchable arm arrangement having more than three arms.